High-purity sponge titanium material and its production method

ABSTRACT

The object of the present invention is to economically produce a high-purity sponge titanium material containing fewer amounts of oxygen and metal elements. To realize this object the vacuum separation time t in a vacuum separation step is t=t o +(15˜35) hour where the time t is the vacuum separation time in a vacuum separation step and t o  is defined as the time from the start of the vacuum separation till the time when the temperature of the central part of the material in a reaction vessel reaches a stable temperature. At and near the central part of the material where the amounts of metal elements are small, the specific area measured by the BET method is 0.05 m 2 /g or less. Thus, even if the cutting and crushing of the material are performed in the atmosphere, the amount of oxygen in the cut and crushed material can be suppressed to a low level.

TECHNICAL FIELD

The present invention relates to a high-purity sponge titanium material suitable for a crude material of a sputtering target material and its production method.

BACKGROUND ART

As an industrial production method of metal titanium, a method is often used in which after a sponge titanium material produced by Kroll method was cut and crushed, it is compressed into briquettes, and the briquettes are melted and cast. The Kroll method comprises a reduction step in which molten Mg and titanium tetrachloride are reacted with each other in a reaction vessel and a vacuum separation step in which after the reduction step, non-reacted Mg contained in the material in the vessel and remaining by-products are vaporized to be removed by heating them under vacuum.

On the other hand, as a new use of metal titanium, a wiring material in a semiconductor element such as an LSI or the like is known. This wiring is formed by sputtering using high-purity metal titanium as a target material. Lesser impurities are required for this sputtering target material. Thus, in a sponge titanium material which is a crude material of the target material, oxygen content of 200 ppm or less and each content of metal elements of Ni, Cr, Al and Si of 10 ppm or less are required.

However, since Kroll method is the one, which gives high priority in productivity, it is not easy for the method to ensure such impurity levels as required in the sputtering target material. Thus, following two methods are proposed. One of them is a center selection method in which after vacuum separation, a part at and near the center of a sponge titanium material obtained in a reaction vessel other than the upper part, lower part and the outer circumferential part thereof is commercialized (Japanese Patent No. 2863469). The other method is a low humidity crushing method in which a sponge titanium material taken out of the reaction vessel is cut and crushed under low humidity atmosphere (Japanese Patent No. 2921790).

However, although in the former center selection method, it is possible for the respective metal elements of Fe, Ni, Cr, Al and Si to ensure the level required for the sputtering target material, it is impossible for oxygen to ensure the level. On the other hand, in the latter humidity crushing method, it is possible for oxygen to ensure the level required for the sputtering target material, it is impossible for the respective metal elements of Fe, Ni, Cr, Al and Si to ensure the level.

Therefore, to ensure the level required for the sputtering target material in both of the respective metal elements of Fe, Ni, Cr, Al and Si, and oxygen, it becomes necessary to combine the former, center selection method, with the latter, low humidity crushing method. However, as an actual problem in a case of the former, implementation of center selection method is easy, but in a case of the latter, low humidity crushing method, to ensure the oxygen level actually required for the sputtering target material stably, a very large partitioned working space which is maintained at low humidity is needed whereby very large costs are required for the construction of the space and the maintenance of atmosphere. These methods are not practical.

The object of the present invention is to provide a high-purity sponge titanium material having both small contents of oxygen and metal elements and which is excellent in cost effectiveness and its production method.

DISCLOSURE OF THE INVENTION

To attain the above-mentioned objects, the present inventor has noted relationships between sample taking positions and oxygen contents for sponge titanium material made by Kroll method.

FIG. 1 is a vertical cross-sectional view of a sponge titanium material in a reaction vessel in a vacuum separation step. A reaction vessel 20 is contained in a heating furnace 30. A sponge titanium material 10 in the reaction vessel 20 has a constricted shape in the middle part since titanium is precipitated on a grate 21 in the reaction vessel 20 and on an inner surface of a side wall in the reaction vessel 20 in the preceding reduction step.

In the sponge titanium material 10 obtained after the vacuum separation step, the respective metal elements of Fe, Ni, Cr, Al and Si have smaller contents in portions further distant from the upper surface, the lower surface and the outer circumferential surface of the reaction vessel 20. This is because the inclusion of the respective metal elements is mainly due to contamination from the reaction vessel 20. Thus, while removing the upper part, the lower part and the outer circumferential part of the sponge titanium material 10, the remaining part 11 of the material near the center is taken so that levels of metal elements required for the sputtering target material can be comparatively easily ensured.

However, an oxygen content, particularly, an oxygen content after cutting and crushing is unexpectedly reduced in a surface layer region of the sponge titanium material 10. For example, examining a distribution of the amount of oxygen in each part after cutting and crushing sponge titanium material 10 from the uppermost part A to the ½ part C at the centerline position, the amount of oxygen increases more as approaching the ½ part C. For example in a case where an oxygen content of titanium particles obtained from the uppermost part A is 250 ppm, the amount of oxygen in the ¼ part B reaches about 300 ppm and the amount of oxygen in the ½ part C reaches about 350 ppm. For this tendency of the amount of oxygen, even if a part 11 near the center of the sponge titanium material 10 is taken, it becomes impossible for the amount of oxygen in a cut and crushed material to ensure the level required for a sputtering target material.

The present inventor has examined reasons why the amount of oxygen is reduced at a surface layer region of the sponge titanium material 10 from both aspects of the physical properties and production method. As a result the following facts have been found.

FIG. 2 shows changes in temperatures of a sponge titanium material in a vacuum separation step, with respect to the uppermost part A, the ¼ part B and ½ part C at the centerline position of the material. The temperatures of each part have a tendency to be temporarily lowered from the start of vacuum separation, and increased, and reach stable temperature near the furnace temperature. This reason is that although the vaporization of remaining Mg is started together with the start of the vacuum separation, and temperature is temporarily lowered by its heat of vaporization, an increase in the temperature is started by reduction in remaining Mg and a temperature is stabilized at a level near the furnace temperature.

This tendency is common even in any part of the uppermost part A, the ¼ part B and the ½ part C. Nevertheless, the lowest temperature goes down in the order of the uppermost part A, the ¼ part B and the ½ part C, and both time from the start of the vacuum separation to the turning point to an increase in temperature and time from the increase in temperature to a stabled temperature are further increased in the order of the uppermost part A, the ¼ part B and the ½ part C. This reason is that remaining Mg is difficult to be removed at a part nearer the center of the sponge titanium material. Thus the time when remaining Mg in the central part where it is difficult to be removed, that is the ½ part C, is removed and temperature Tc in this part reaches a stable temperature To is the finish time of vacuum separation. It is noted that the reason why stable temperatures become lower in the order of the ½ part C, the ¼ part B and the uppermost part A is that heat radiation toward an opening part side (upper side) of the reaction vessel is remarkable.

As a result of vacuum separation in such a process, in a surface layer region far from the center of the sponge titanium material, heating is continued for a long period of time even after the vaporization of Mg was finished, resulting in so called conditions of heating an empty object. The present inventor has considered that the heating time after vaporization of Mg has relation to the oxygen content after cutting and crushing the sponge titanium material, and examined variously. As the results, the present inventor has found that at a position farther from the center of the sponge titanium material, sintering is further advanced while heating under conditions of heating empty object whereby specific area is decreased; in a part having smaller specific area is suppressed an increase in oxygen content due to oxidation in cutting and crushing steps; as a result, the amount of oxygen in cut and crushed titanium material becomes lower in the order of the ½ part C, the ¼ part B and the uppermost part A; and if, after the completion of vaporization of Mg in the ½ part C, heating is continued, sintering of this part is advanced and specific area is decreased so that low oxygen can be realized.

The present invention has been completed based on these knowledge and is a high-purity sponge titanium material produced by Kroll method wherein a specific area measured by the BET method is set at 0.05 m²/g or less and the content of respective metal elements of Fe, Ni, Cr, Al and Si is set at 10 ppm or less.

By limiting the specific area of the material measured by the BET method to 0.05 m²/g or less, even if cutting and crushing the material under usual atmosphere the amount of oxygen of the titanium particle is suppressed to 300 ppm or less. Preferably the specific area is 0.04 m²/g or less, and 0.03 m²/g or less is more preferably. This reduces the amount of oxygen. Preferable oxygen content after cutting and crushing is 200 ppm or less, and more preferably is 100 ppm or less. As for the lower limit of the specific area, it is better to be lower from a viewpoint of reduction in oxygen content. However, when the specific area is too small, cutting and crushing the material is difficult. Thus the specific area is preferably 0.01 m²/g or more.

The reason why the respective metal elements of Fe, Ni, Cr, Al, and Si were limited to 10 ppm or less is to exclude the upper part, the lower part and the outer circumferential part of a sponge titanium material. The surface layer region of these parts, for example an uppermost part A shown in FIG. 1 receives heating in conditions of heating an empty object so that a specific area measured by the BET method is decreased. However, in this part the contents of the respective metal elements exceed 10 ppm. Meantime, particularly preferable content of the respective metal elements is 7 ppm or less.

Further, a production method of a high-purity sponge titanium material of the present invention is one when a sponge titanium material is produced by Kroll method, vacuum separation time t in a vacuum separation step is set at t=t_(o)+(15˜35) hour while defining t_(o) as the time from the start of vacuum separation till the time when the temperature Tc of the central part of the material in a reaction vessel reaches a stable temperature To near the furnace temperature, and after the completion of vacuum separation, a part at and near the central part other than the upper part, the lower part and the outer circumferential part of the material in the reaction vessel is commercialized.

By setting the vacuum separation time t as (t_(o)+15) hour or more a specific area of a part at and near the center other than the upper part, the lower part and the outer circumferential part of the material in the reaction vessel is decreased so that low oxygen after crushing and crushing material is realized. And by commercializing the part near the center other than the upper part, the lower part and the outer circumferential part of the material in the reaction vessel the contents of the respective metal elements of Fe, Ni, Cr, Al and Si are suppressed to low levels.

When the vacuum separation time t exceeds (t_(o)+35) hour, the specific area becomes too small to cut and crush the material. Further, thermal cost efficiency further becomes deteriorated unnecessarily. A particularly preferable lower limit of the vacuum separation time t is (t_(o)+20) hour or more, and a preferable upper limit thereof is (t_(o)+30) hour or less.

It is noted that in an actual operation, the temperature of the central part in a sponge titanium material is not measured. Time when the temperature of the central part of the material should reach a stable temperature is assessed from data of changes in temperatures obtained by a test operation and a temperature analysis every operating plant, and heating time in the vacuum separation step is set with reference to this time.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a vertical cross-sectional view of a sponge titanium material in a reaction vessel in a vacuum separation step, FIG. 2 is a graph showing time-varying changes in temperatures of sponge titanium materials in a vacuum separation step with respect to the uppermost part A, ¼ part B, and ½ part C, FIG. 3 is a graph showing preferable vacuum separation time in the vacuum separation step using a diameter of the reaction furnace (retort diameter) as a parameter, and FIG. 4 is micrographs of samples taken from the central part of two kinds of sponge titanium materials, which are different from vacuum separation time.

BEST MODE FOR CARRYING OUT THE INVENTION

Embodiments of the present invention will be described.

Mg (Magnesium) is melted in a reaction vessel and a titanium tetrachloride solution is dropped, and thereby produces a sponge titanium material. When this reduction step is over, it is transferred to a vacuum separation step. In the vacuum separation step the inside of the reaction vessel is made in vacuum conditions and the sponge titanium material is heated at a predetermined temperature by a heating furnace whereby non-reacted Mg and by-products are removed.

This vacuum separation step will be described in detail with reference to examples of test operations shown in FIG. 2. Temperature Ta of the uppermost part A of the sponge titanium material in the reaction vessel is slightly lowered at the start of the vacuum separation. However, this temperature is immediately increased and reaches a stable temperature for about 20 to 30 hours from the start of the vacuum separation. On the other hand, temperature Tc of the central part (½ part C) continues to become lower for about 30 hours from the start of the vacuum separation, and after that, the temperature starts rising and reaches to a stable temperature To after 70 hours from the start of the vacuum separation.

The vacuum separation has been conventionally finished for about 70 hours when the temperature Tc of the central part reaches a stable temperature To. Although a trial of reducing this time was performed, but a trial of extending this time has not been considered. As a result, since heating the material after vaporization of Mg in conditions of heating an empty object is not performed in a part at and near the central part where contents of metal elements are small and sintering does not advance, a specific area is not sufficiently decreased. Thus even if a part at and near the center of the material is taken after vacuum separation, an increase in oxygen content by oxidation becomes remarkable in cutting and crushing the material in the atmosphere and it was impossible to realize a low oxygen content's level required for a sputtering target material.

Thus, in this embodiment, after the temperature Tc of the central part reached the stable temperature To, heating is further continued for 15 to 35 hours, preferably for 20 to 30 hours. Thus, sintering is advanced even in a part at and near the center of the material having small contents of metal elements whereby a specific area by the BET method is reduced to 0.05 m²/g or less. As a result, when the part near the center of the material is taken after vacuum separation, an increase in oxygen content due to oxidation in the cutting and crushing steps is suppressed so that in both of oxygen content and contents of metal elements it becomes possible to realize impurity levels required for the sputtering target material.

In taking material from the part near the center of a sponge titanium material 10 after vacuum separation, following three parts of the material 10 are cut and removed as shown in FIG. 1. The first part is an upper part having a thickness h1 from the upper surface of 0.1 H or more, the second part is a lower part having a thickness h2 from the bottom surface of 0.25 H or more, and the third part is an outer circumferential part having a thickness d from the outer circumferential surface of 0.18 D or more. In this case the height of the sponge titanium material mass is defined as H, and the mass diameter thereof is defined as D. After the cutting and removing the three parts of the material, a remaining part 11 near the center, having less than 30% of the mass weight of the sponge titanium material 10, is taken.

The selected part 11 near the center is usually cut and crushed in the atmosphere to make sponge titanium particles each having a predetermined particle size. In spite of usually cutting and crushing in the atmosphere, the amount of oxygen is suppressed to 300 ppm or less and the content of the respective Fe, Ni, Cr, Al and Si is suppressed to 10 ppm or less. The particle size of crushed material is preferably 10 to 300 mm in average.

A result of examination of preferable vacuum separation time in a vacuum separation step is shown in FIG. 3. A preferable vacuum separation time is in a region shown by the slanted lines in FIG. 3.

The vacuum separation time receives the influence of a diameter (retort diameter) of a reaction vessel, and the larger this diameter, the time becomes longer. Conventional vacuum separation time is shown by a solid line in FIG. 3. The vacuum separation time of the present invention is +15 to +35 hours to the conventional vacuum separation time shown by a solid line. A specific area of the part near the center of the material by the BET method reaches 0.05 m²/g or less for +15 hours or more, and the amount of oxygen after cutting and crushing material reaches 300 ppm or less. Further, a specific area of the part near the center of the material by the BET method reaches 0.03 m²/g or less for +20 hours or more, and the amount of oxygen after cutting and crushing material reaches 200 ppm or less. In a case where the specific area by the BET method is less than 0.01 m²/g, it is impossible to cut and crush the material.

A diameter of the reaction vessel (retort diameter) is preferably 1350 to 2000 mm. In a case of the diameter of less than 1350 mm, even if selection of the part near the center of the material is performed, metal impurities have a tendency to increase. In a case where the diameter exceeds 2000 mm there can be generated a problem in facilities such as thermal deformation or the like.

It is noted that in the case of change in temperature shown in FIG. 2, the diameter (retort diameter) of the reaction vessel is 1700 mm. The time for a conventional vacuum separation is 70 hours and the time for the present invention in this case is 85 to 105 hours, and preferably 90 to 100 hours.

In the case where a diameter (retort diameter) of the reaction vessel is 1700 mm, micrographs of samples taken from the central part of a sponge titanium material are shown in FIGS. 4(a) and 4(b) by the same magnification with respect to cases of 70 hours and 90 hours for vacuum separation time respectively.

A specific area by the BET method is 0.1 m²/g in the case of 70 hours of vacuum separation time, and it is 0.03 m²/g in the case of 95 hours of vacuum separation time. This difference is clear from FIGS. 4(a) and 4(b). As the results of the difference between these specific areas the amount of oxygen of cut and crushed materials having average particle size of 65 mm in the atmosphere reached 320 ppm in a case of 70 hours of vacuum separation time and 190 ppm in a case of 95 hours of vacuum separation time. Further, the contents of the respective metal elements of Fe, Ni, Cr, Al and Si reached 10 ppm or less in all cases.

It is noted that the region shown by slanted lines in FIG. 3 is as follows if it is expressed by a numerical formula.

Vacuum separation time=(0.0698×[retort diameter, mm]−24)±10

The BET method is a method for calculating a specific area from an adsorption of liquid nitrogen and is widely used in adsorbent and the like.

INDUSTRIAL APPLICABILITY

As described above, in a high-purity sponge titanium material of the present invention, by limiting the specific area measured by the BET method to 0.05 m²/g or less, the amount of oxygen can be suppressed to a low level even if cutting and crushing are carried out in the atmosphere. Further, impurity levels required for the sputtering target material can be economically ensured by the reduction of metal impurities by selecting the central part of the material.

Further, in a production method of the high-purity sponge titanium material of the present invention, by setting the vacuum separation time t in a vacuum separation step at t=t_(o)+(15˜35) as the time t_(o) is defined from the start of the vacuum separation till the time that the temperature Tc of the central part of the material in a reaction vessel reaches to a stable temperature To, the specific area measured by the BET can be easily limited to 0.05 m²/g or less. And by a decrease in the amount of oxygen due to the limitation and reduction of metal impurities obtained by selecting the central part of the material, impurity levels required for the sputtering target material can be economically ensured. 

1. A high-purity sponge titanium material produced by Kroll method wherein a specific area measured by the BET method is set at 0.05 m²/g or less and the content of respective metal elements of Fe, Ni, Cr, Al and Si is set at 10 ppm or less.
 2. A production method of a high-purity sponge titanium material of the present invention is one in which when a sponge titanium material is produced by Kroll method, the vacuum separation time t in a vacuum separation step is set at t=t_(o)+(15˜35) hour while defining t_(o) as the time from the start of vacuum separation till the time when the temperature Tc of the central part of the material in a reaction vessel reaches a stable temperature To near the furnace temperature, and after the completion of vacuum separation, a part at and near the central part other than the upper part, the lower part and the outer circumferential part of the material in the reaction vessel is commercialized.
 3. A high-purity sponge titanium material according to claim 1 wherein the amount of oxygen of cut and crushed high purity sponge titanium material is 200 ppm or less.
 4. The production method of the high-purity sponge titanium material according to claim 2, wherein said high-purity sponge titanium material has a specific area measured by the BET method of 0.05 m²/g or less and a content of respective metal elements of Fe, Ni, Cr, Al and Si of 10 ppm or less.
 5. The production method of claim 4, wherein an amount of oxygen of cut and crushed high purity sponge titanium material is 200 ppm or less. 